The present disclosure relates to a fixing device fixing a toner image on a sheet and an image forming apparatus including this fixing device.
As one manner heating a fixing belt of a fixing device, a manner using a flat heater is known. The flat heater has a heat generating part having a longitudinal direction orthogonal to a conveying direction of a sheet, and the cylindrical fixing belt is sandwiched between the flat heater and a pressuring roller. When the pressuring roller is driven and rotated, the fixing belt is co-rotated with the pressuring roller, and the sheet on which the toner is transferred is sandwiched between the fixing belt and the pressuring roller and conveyed, and then, the toner is fixed on the sheet.
The above-mentioned heat generating part has a length corresponding to a sheet of a maximum size as a size in a direction orthogonal to the conveying direction, but in a case where a sheet with a size smaller than it is used, heat consumption of both ends in the longitudinal direction through which the sheet does not pass is decreased. Thereupon, a configuration is proposed, in which heat generating part is divided into a plurality of heat generating parts and the heat generating part according to the size of the sheet is heated. However, in a case providing a safety device (e.g. thermostat) interrupting power supply to the heat generating part when abnormal temperature rise occurs, if the safety devices are provided for each of the heat generating parts, cost is increased. Moreover, because wirings are increased, compact design is difficult.
However, in the fixing device having an above-described configuration, because the thermostat is provided only to the heating area at the center in the longitudinal direction, even if abnormal temperature rise occurs at the other heating area, power supply cannot be interrupted.